Most battery-operated appliances are designed to operate at a low voltage, "low voltage" indicating less than 50 volts. Such appliances include portable computers, calculators, radios, televisions, and hand tools. In order to operate from AC household current, such an appliance is often equipped with a current-limiting circuit such as a wound-wire transformer, even though such transformers are bulky, heavy, and expensive. It is believed that all brushless DC motors have required transformers to be operated from AC. Some small motors, called universal motors, employ brushes and a commutator to permit operation directly from either AC or DC. Although avoiding the need for a current-limiting circuit, a universal motor is noisy both acoustically and electrically and has relatively short life due to wear of the brushes against the commutator. Moreover, they tend to be expensive to manufacture in very small sizes.
Discharge lamps such as fluorescent lamps need a current-limiting circuit, usually called a "ballast" and typically including one or more transformers and reactors. Since the commercial introduction of fluorescent lamps some 40-50 years ago, their current-limiting circuits have become more efficient and less expensive, but invariably still have energy losses associated with the hysteresis and eddy currents in the laminated steel cores of their transformers.
High-intensity, low-voltage lamps which operate from AC household current also require current-limiting, voltage-reducing circuits, invariably comprising a wound-wire transformer.
Some low-voltage appliances such as radios have employed as a current-limiting circuit a power resistor to permit operation directly from AC household current through a power resistor, but because such resistors are expensive and highly inefficient, they are no longer used for that purpose.